Mars lies outside Earth's orbit, so it traverses the entire ecliptic plane, as seen from Earth. Its orbit is more elliptical than Earth's, so its distance from the Sun varies more. Mars rotates at almost the same rate as Earth, and its rotation axis is inclined to the ecliptic at almost the same angle as Earth's axis. Because of its axial tilt, Mars has daily and seasonal cycles much like those on our own planet, but they are more complex than those on Earth because of Mars's eccentric orbit.
From Earth, the most obvious Martian surface features are the polar caps, which grow and diminish as the seasons change on Mars. The appearance of the planet also changes because of seasonal dust storms that obscure its surface.
Like the atmosphere of Venus, Mars's atmosphere is composed primarily of carbon dioxide. However, unlike Venus's, the density of the cool Martian atmosphere is less than 1 percent that of Earth's. Mars may once have had a dense atmosphere, but it was lost, partly to space and partly to surface rocks and subsurface permafrost and polar caps. Even today, the thin atmosphere is slowly leaking away. Surface temperatures on Mars average about 50 K cooler than those on Earth. Otherwise, Martian weather is reminiscent of that on Earth, with dust storms, clouds, and fog.
The two polar caps on Mars consist of a seasonal cap, composed of carbon dioxide, which grows and shrinks, and a residual cap, of water ice, which remains permanently frozen.
There is a marked difference between the two Martian hemispheres. The northern hemisphere consists of rolling volcanic plains and lies several kilometers below the level of the heavily cratered southern hemisphere. The lack of craters in the north suggests that this region is younger. The cause of the north—south asymmetry is not known.
In 1971, Mariner 9 mapped the entire Martian surface, revealing plains, volcanoes, channels, and canyons. Viking 1 and Viking 2 reached Mars in 1976 and returned a wealth of data on the planet's surface and atmosphere. Experiments onboard the Viking landers detected no evidence for Martian life.
Martian craters differ from those on the Moon by the presence of fluidized ejecta, which provide direct evidence for the permafrost layer beneath the surface.
Mars's major surface feature is the Tharsis bulge, located on the planet's equator. It may have been caused by a "plume" of upwelling material in the youthful Martian mantle. Associated with the bulge is Olympus Mons, the largest known volcano in the solar system, and a huge crack, called the Valles Marineris, in the planet's surface. The height of the Martian volcanoes is a direct consequence of Mars's low surface gravity. No evidence for recent or ongoing eruptions has been found.
There is clear evidence that water once existed in great quantity on Mars. Mars may have had a brief "Earthlike" phase early on in its evolution. The runoff channels are the remains of ancient Martian rivers. The outflow channels are the paths taken by flash floods that cascaded from the southern highlands into the northern plains. Today, a large amount of that water may be locked up in the polar caps and in the layer of permafrost lying under the Martian surface.
Convection in the Martian interior seems to have been stifled 2 billion years ago by the planet's rapidly cooling and solidifying mantle.
Mars has an extremely weak magnetic field. Since the planet rotates rapidly, this implies that its core is nonmetallic, nonliquid, or both. The lack of current volcanism, the absence of any significant magnetic field, the planet's relatively low density, and a high abundance of surface iron all suggest that Mars never melted and differentiated as extensively as did Earth.
Mars's moons, Phobos and Deimos, are probably asteroids captured by Mars early in its history. Their densities are far less than that of any planet in the inner solar system. They may be representative of conditions in the early solar system.
1. It is possible, over time, to see Mars at any angular distance from the Sun, at any time of night. HINT
2. Seen from Earth, Mars goes through phases, just like Venus and Mercury. HINT
3. Mars at times is engulfed by global dust storms. HINT
4. Because Mars has such a thin atmosphere, the planet has no significant surface winds. HINT
5. Seasonal changes in the appearance of Mars are caused by vegetation on the surface. HINT
6. Mars has the largest volcanoes in the solar system. HINT
7. The northern hemisphere of Mars is much older than the southern hemisphere. HINT
8. Olympus Mons is the largest impact crater on Mars. HINT
9. There are many indications of past plate tectonics on Mars. HINT
10. Valles Marineris is similar in size to Earth's Grand Canyon. HINT
11. The orange-red color of the surface of Mars is primarily due to rust (iron oxide) in its soil. HINT
12. Daytime temperatures on Mars can reach 300 K. HINT
13. The Viking landers are still sending data back to Earth.
14. No spacecraft have landed on Mars since the 1970s. HINT
15. One of the two moons of Mars is larger than Earth's Moon. HINT
1. When Mars is on the opposite side of the Sun from Earth, it is said to be at _____. HINT
2. The radius of Mars is about ____ that of Earth. (Give an answer as a simple fraction.) HINT
3. The length of the Martian day is about _____ hours. HINT
4. The seasonal polar ice caps of Mars are composed of _____. HINT
5. The southern hemisphere of Mars consists of heavily _____ highlands. HINT
6. The Tharsis region of Mars is a large equatorial _____. HINT
7. Several large _____ lie at approximately the center of Tharsis. HINT
8. The great height of Martian volcanoes is a direct result of the planet's low _____. HINT
9. The fluidized ejecta surrounding Martian impact craters is evidence of a layer of _____ just under the surface. HINT
10. Runoff channels carried _____ from the southern mountains into the valleys. HINT
11. Outflow channels are the result of catastrophic _____. HINT
12. Water flowed on the surface of Mars a few _____ years ago. HINT
13. Mars's northern residual polar ice caps consists mostly of _____. HINT
14. Most of the carbon dioxide on Mars is now found in the planet's _____. HINT
15. The most recent claims of life on Mars have been based on analyses of _____ found on Earth. HINT
1. When is the best time to see Mars from Earth? HINT
2. What is the evidence that water once flowed on Mars? HINT
3. Is there water on Mars today? HINT
4. For a century, there was speculation that intelligent life had constructed irrigation canals on Mars. What did the "canals" turn out to be? HINT
5. Imagine that you will be visiting the southern hemisphere of Mars during its summer. Describe the atmospheric conditions you might face. HINT
6. Describe the two Martian polar caps, their seasonal and permanent composition, and the differences between them. HINT
7. Why is Mars red? HINT
8. Why couldn't you breathe on Mars? HINT
9. Why were Martian volcanoes able to grow so large? HINT
10. How were the masses of Mars's moons measured, and what did these measurements tell us about their origin? HINT
11. What is the evidence that Mars never melted as extensively as did Earth? HINT
12. How would Earth look from Mars? HINT
13. If humans were sent to Mars to live, what environmental factors would have to be considered? What resources might Mars provide, and which would have to come from Earth?
14. Since Mars has an atmosphere, and it is composed mostly of carbon dioxide, why isn't there a significant greenhouse effect to warm its surface?
15. Compare and contrast the evolution of the atmospheres of Mars, Venus, and Earth.
1. Verify that the surface gravity on Mars is 40 percent that of Earth. HINT
2. Calculate Earth's synodic period, as seen from Mars. HINT
3. What is the maximum elongation of Earth, as seen from Mars? (For simplicity, assume circular orbits for both planets.) HINT
4. A certain star, observed from Earth, has a parallax of 0.1". What would be its parallax as seen from Mars? How might observations from Mars-based telescopes be superior to those made from Earth? HINT
5. Using the data from Section 10.7, compute the synodic periods of the moons Phobos and Deimos, as seen from the Martian surface (see More Precisely 9-1). HINT
6. How long would it take the wind in a Martian dust storm, moving at a speed of 150 km/h, to encircle the planet? HINT
7. The mass of the Martian atmosphere is about 1/150 the mass of Earth's atmosphere and is composed mainly (95 percent) of carbon dioxide. Using the result of problem 3, Chapter 7, to determine the mass of Earth's atmosphere, estimate the total mass of carbon dioxide in the atmosphere of Mars. Compare this with the mass of a seasonal polar cap, approximated as a circular sheet of frozen carbon dioxide ("dry ice," having a density of 1600 kg/m3) of diameter 3000 km and thickness 1 m.
8. Assume that a planet will have lost its initial atmosphere by the present time if the molecular speed exceeds one-sixth of the escape speed (see More Precisely 8-1). What would the mass of Mars have to have been in order for it to have retained an Earthlike atmosphere?
9. The outflow channel shown in Figure 10.9 is about 10 km across and 100 m deep. If it carried 107 metric tons (1010 kilograms) of water per second, as stated in the text, estimate the speed at which the water must have flowed. HINT
10. Using the data given in the text, calculate the maximum angular sizes of Phobos and Deimos, as seen by an observer standing on the Martian surface directly under their orbits. Compare these sizes with the angular diameter of Earth's Moon, as seen from Earth. Would a Martian observer ever see a total solar eclipse? HINT
1. Track the motion of the Red Planet in front of the stars for several months following its return to the predawn sky. (Consult an almanac to determine where Mars will be in the sky this year.) You will see that Mars moves rapidly in front of the stars, crossing many constellation boundaries.
2. Several months before opposition, Mars begins retrograde motion. Chart the planet's motion in front of the stars to determine when it stops moving eastward and begins moving toward the west.
3. Notice the increase in Mars's brightness as it approaches opposition. Why is it getting brighter? What other planets appear in the sky now? How do their brightnesses compare with that of Mars?
4. Look at Mars with as large a telescope as is available to you; binoculars will not be of use. Prepare ahead of time and find out the Martian season that will be occurring at the time of your observation, which hemisphere will be tilted in Earth's direction, and what longitude will be pointing toward Earth at the time of observation. This information can be obtained from many different computer almanacs. Sketch what you see. Look very carefully and take your time. Afterward, try to identify the various features you have seen with known objects on Mars.
